Many thin steel sheets are used as materials for automobile bodies. Cold-rolled steel sheets used to be used for applications in which superior formability is required. However, owing to adjustment of steel compositions (chemical constituents) and optimization of hot rolling conditions, it has become possible to produce hot-rolled steel sheets having high formability (high workability), and therefore, the hot-rolled steel sheets are increasingly used as materials for automobile bodies.
In order to meet restrictions on exhaust gas in view of the global environment, reductions in automobile body weight are very important. In order to reduce the automobile body weight, it is effective to increase the tensile strength of steel sheets and decrease the thickness of the steel sheets. Automotive components to which higher tensile strength and thinner steel sheets are applied must have various characteristics. For example, the required characteristics include static strength to bending and torsional deformation, fatigue strength, and impact resistance. Therefore, the high tensile strength steel sheets used for the automotive components must have such characteristics after formation and working are performed.
On the other hand, press forming is performed to steel sheets when automotive components are manufactured. Excessively high strength of the steel sheets gives rise to problems; for example, shape fixability is degraded, and defects, such as cracking and necking, are caused during formation due to a decrease in ductility. Such problems have hindered the expansion of the application of high tensile strength steel sheets to automobile bodies.
In order to overcome the difficulties described above, for example, with respect to cold-rolled steel sheets for outer panels, a steel sheet production technique is known in which an ultra low-carbon steel is used as a raw material and the C amount ultimately remaining in the dissolved state is restricted within an appropriate range. In this technique, a strain aging hardening phenomenon, which occurs in a paint baking step performed at 170° C.× approximately 20 minutes after press forming, is used. Shape fixability and ductility are secured during formation by maintaining the softness, and dent resistance is secured after formation by an increase in YS (yield stress) due to strain aging hardening. However, in this technique, in order to avoid stretcher strain leading to surface defects, an amount of the increase in YS cannot be increased sufficiently, and since ΔTS is as small as several Mpa, the thickness of the steel sheet cannot be decreased sufficiently.
On the other hand, in the applications in which appearance is not a great problem, a steel sheet in which the bake hardening amount is further increased by using dissolved N (Japanese Examined Patent Application Publication No. 7-30408), and a steel sheet in which bake hardenability is further improved by using a dual-phase structure composed of ferrite and martensite (Japanese Examined Patent Application Publication No. 8-23048) have been disclosed.
However, in such steel sheets, although a higher bake hardening amount can be obtained because YS (yield stress) is increased to a certain extent after paint baking, it is not possible to increase TS (tensile strength), and no great improvement in fatigue resistance and impact resistance after formation is expected. Therefore, the steel sheets cannot be used for components in which fatigue resistance, impact resistance, etc., are required, which is disadvantageous. Since the amount of the increase in the yield stress YS is unstable, it is not possible to decrease the thickness of the steel sheets in such a way as to contribute to lightening of automotive components, which is also disadvantageous.
Moreover, when a thin steel sheet with a thickness of 2.0 mm or less is produced, since the shape of the steel sheet becomes unsatisfactory in the hot rolling process, it is considerably difficult to press-form the steel sheet.
It would therefore be advantageous to provide a high tensile strength hot-rolled steel sheet having superior strain aging hardenability which overcomes the limitations of the conventional techniques described above, which has high formability and stable quality characteristics, and in which satisfactory strength is obtained when the steel sheet is formed into automotive components, thus greatly contributing to lightening of automobile bodies. It would also be advantageous to provide a method for industrially producing such a steel sheet at low costs and without disturbing the shape thereof.